1. Field of the Invention
This invention relates to a common-rail fuel injection system for an engine.
2. Description of the Prior Art
Common-rail fuel injection systems for diesel engines are disclosed in various documents such as Japanese published unexamined patent application 65-258160, Japanese published unexamined patent application 2-176158, European published patent application 0307947-A2, U.S. Pat. No. 4,777,921, and U.S. Pat. No. 4,940,034.
The common-rail fuel injection systems include a high pressure tubing which forms a pressure accumulator referred to as "a common rail". The fuel injection systems of this type also include high pressure fuel supply pumps for feeding high pressure fuel to the common rail, and solenoid valves for selectively allowing the high pressure fuel to flow from the common rail through injectors into engine cylinders.
The high pressure fuel supply pumps in the common-rail fuel injection system include pumping chambers, and movable plungers partially defining the pumping chambers respectively. The plungers are driven by the diesel engine through a suitable mechanism. The drive of the plungers pressurizes fuel in the pumping chambers, forcing the fuel from the pumping chambers into the common rail. In general, spill or relief solenoid valves are connected to the pumping chambers respectively. Closing and opening the relief solenoid valves enables and disables pumping the fuel from the pumping chambers into the common rail. Thus, the rate of fuel supply to the common rail is adjusted by controlling the relief solenoid valves.
The relief solenoid valves are of the normally-open type. The valve members of the relief solenoid valves are designed so that they will be urged by the pressure in the pumping chambers toward their closed positions. When a high pressure pump plunger is required to drive the fuel into the common rail, the related relief solenoid valve is energized to move its valve member to a closed position so that the fuel supply from the pumping chamber to the common rail is enabled. Then, the valve member is held in the closed position by a resulting high pressure in the pumping chamber, and the relief solenoid valve can be de-energized to save electric power. The rate of fuel supply to the common rail is adjusted by controlling the timing of energizing the relief solenoid valve, that is, the timing of closing the relief solenoid valve.
Prior art common-rail fuel injection systems have the following problems. Under overrunning conditions where the crankshaft of an engine rotates at a high speed and the fuel supply to a common rail is required to be inhibited, since the mean speed of movement of plungers in high pressure fuel supply pumps is high, the inertia of fluid in pumping chambers is great and thus relief solenoid valves tend to be closed by the fluid inertia even in the absence of relief solenoid valve energizing signals. Closing the relief solenoid valves results in unwanted fuel supply to the common rail. Such unwanted fuel supply to the common rail tends to cause an excessively high pressure in the common rail and a damage to the common rail.